1. Field Of The Invention
This invention relates to novel vinyl chloride resins and, more particularly, to novel resins for use in compositions to be used in magnet recording media, enhancing the dispersion and magnetic property characteristics of such medium.
2. Description Of The Prior Art
Over the years, magnetic recording has come to occupy a vital place in a myriad of industries. Magnetic tape is accordingly utilized for audio, video, computer, instrumentation and other recordings. Magnetic recording media are utilized in a variety of forms, including, for example, magnetic cards and disks, reels, video tapes, high performance audio tapes, computer tapes, floppy disks and the like.
While there are several different types of magnetic recording media, all types consist of a layer of magnetic particles, sometimes referred to as "pigment", coated on a plastic, paper or metal base. Information to be recorded is stored in the magnetic pigment applied to the base as a series of small domains magnetized by a recording head. The coating layer of the magnetic pigment includes a binder system which provides a cohesive matrix between the magnetic pigment particles and adheres such particles to the base.
The magnetic coating is applied to the base by coating equipment such as, for example, a gravure roll coater, and the coated base then typically immediately proceeds to a magnetic orientation step wherein orientation of the pigment particles is effected on the undried layer. In this step, the long axis of the pigment particles, typically acicular crystals, is made to coincide with the magnetization direction.
In order to achieve good recording performance, the magnetic coating must possess a wide variety of characteristics. Pigment particles, desirably of relatively uniform particle size, should form as high a proportion of the coating layer as possible. Further, the degree of dispersion of the pigment particles in the coating, often evaluated as degree of gloss, should be as high as possible. Further, the highly dispersed pigment particles must be capable of being adequately oriented as previously described (the degree of orientation often measured as "squareness").
Still further, the adhesion and wear resistance of the magnetic coating or film should be high. Also, the coefficient of friction of the magnetic surface should be low against the head material, and yet have an adequate value against the driving media such as pitch rollers and capstan rollers.
Satisfying these and other diverse criteria have proven to require a delicate balance of basically reciprocal, or opposing, properties. A substantial amount of effort over the years has been directed to improving the various characteristics of magnetic recording media.
To satisfy the performance criteria, the binder system must possess adequate modulus, yet have satisfactory tensile strength and resilience. It has generally been found desirable to satisfy these criteria by utilizing more than one material in the binder system. Typically, a polymer of relatively high molecular weight which is capable of being cross-linked or cured is utilized to provide the desired modulus. Various elastomers are also incorporated to achieve the desired resilience, toughness and durability.
Conventional binder systems include a wide variety of high Tg (viz. - glass transition temperature) hardening polymers or resins, such as, for example, polyacrylates, polyesters, polyester amides, polyhydroxyethers and copolymers from monomers such as vinyl chloride, vinyl acetate, acrylonitrile, vinyl alcohol, vinyl butyral, and vinylidene chloride in combination with low Tg elastomeric polymers, including nitrile rubbers, polyesters, alkyd resins and polyester polyurethanes. The latter elastomers often represent the material of choice for high performance applications. Such resins have excellent toughness and abrasion resistance. Typically, the hardening polymers contain hydroxyl functionality since crosslinking to further increase the modulus, durability and abrasion resistance characteristics can then be achieved by using polyfunctional isocyanates.
The degree of dispersion and the capability of the pigment particles to be oriented in many binder systems are often considered inadequate in the absence of dispersion aids. A wide variety of techniques have been proposed to improve these characteristics. Thus, many surface active agents have been employed for this purpose. These include higher aliphatic amines, higher fatty acids, phosphoric acid esters of higher alcohols such as polyoxyethylene phosphate alkyl ethers, esters of higher fatty acids and sorbitol, sodium alkylbenzenesulfonate, betaine-type, nonionic surface active agents and the like.
Magnetic coatings obtained by employing such surface active agents generally exhibit uniform dispersion of magnetic powder as well as good orientation, when the powder or pigment is in the form of acicular particles. On the other hand, utilization of such agents often adversely affects wear resistance or causes difficulties in quality during use. These problems are often attributed to migration or blooming of such surface active agents to the magnetic coating surface or to unnecessary plasticization of the hardening resin or of other components in the magnetic coating.
One attempt to obviate the migration problem is suggested by Great Britain Patent No. 2,097,415A. This suggests using with a cross-linkable or polymerizable resin binder a phosphoric acid ester having at least one polymerizable unsaturated double bond. Preferably, the phosphoric acid ester is a product obtained by reacting a phosphoric acid ester of a compound having two or more hydroxyl groups with an acrylic or methacrylic compound having a functional group reactive with an isocyanate group and also with a polyisocyanate compound. The phosphoric esters disclosed include two major types: (1) the reaction products of a long chain alkylether or polyester, an acrylic or methacrylic compound having a functional group reactive with an isocyanate group, and a polyisocyanate compound with (2) phosphoric esters of hydroxyalkyl acrylates. Other binder systems disclosed utilize compounds containing various hydrophilic groups such as sulfates, sulphonates, hosphonates and the like.
Japanese application No. 116,474 discloses a magnetic layer binder composition of a polyurethane and optionally a polyester containing sulphonate groups, and a vinyl chloride polymer containing sulphonate, sulphate, carboxylate or phosphonate groups. The binder is stated to have good dispersing function to magnetic powder and the recording medium to have excellent service durability and good surface gloss.
Japanese Patent No. J57092421-A(8229) discloses a magnetic recording medium which includes a polyester urethane having (1) at least one hydrophilic group such as --SO.sub.3 M,--OSO.sub.3 H, --COOM, --P(O)(OM').sub.2, --NH.sub.2, --NHR, NR.sub.1 R.sub.2, a bond NR.sub.1 R.sub.2 R.sub.3, wherein M may be H, Li, Na or K; M' may be H, Li, Na, K or a hydrocarbon group; and the R groups may be hydrocarbon groups and (2) at least two acrylic type double bonds.
Japanese No. J57092422-A(8229) discloses a magnetic layer containing a polyurethane or polyester resin having (1) at least one hydrophilic group selected from --OSO.sub.3 H, --COOM, --P(O)(PM').sub.2 wherein M may be H, Li, Na, K and M' may be H, Li, Na, K or a hydrocarbon group and (2) a molecular weight of 200 to 5000 per hydrophilic group.
International Publication No. W08400240-A discloses a magnetic recording medium containing a polyvinyl chloride-vinyl acetate-vinyl alcohol and a polyurethane resin containing polar groups such as --SO.sub.3 M, --OSO.sub.3 M', --COOM or ##STR1## wherein M may be H, Li, Na or K and M.sub.1 and M.sub.2 may be Li, Na, K or an alkyl group, preferably an alkyl group with up to 23 carbon atoms.
Japanese J55117734-A(8043) discloses a binder for a magnetic recording medium which contains a copolymer of a phosphoric acid ester of an alkylene glycol acrylate or an alkylene glycol methacrylate and a copolymerizable monomer. The wetting properties to ferromagnetic material is stated to be very good, so that dispersibility in the binder is improved. Blooming is said not to occur as no high aliphatic acid, metallic soap, or the like is used.
Among the binder polymers in use in magnetic coating media are commercially available, partially hydrolyzed, (viz. - partly saponified) vinyl chloride - vinyl acetate coploymers and terpolymers. Unfortunately, the dispersion and orientation characteristics of coatings utilizing such binder polymers are typically less than is desired. Considerable efforts have been undertaken to improve these characteristics with the use of a wide variety of dispersion aids.
U.S. Pat. No. 4,420,537 to Hayama et al. thus discloses a magnetic recording medium including a commercially available vinyl chloride-vinyl acetate-vinyl alcohol copolymer and a phosphoric ester type anionic surfactant (e.g.--"GAFAC RE 610"). It is noted that when the content of the surfactant is more than 5 weight percent of the coating, the surfactant is bloomed out from the magnetic layer.
U.S. Pat. No. 4,153,754 to Huisman notes difficulties with prior dispersing agents. Low molecular weight agents, such as lecithin, have the disadvantage that an excess is necessary to fully cover the particles to be dispersed. The high molecular weight dispersing agents, as described, for example in Netherlands Patent Application No. 65.11015, have the disadvantage that, due to their poor wetting properties, agglomerates of the particles are also covered with a dispersing agent. It is not readily possible, or it is possible only by the use of much energy, to disintegrate such agglomerates to individual particles. Huisman suggests using an Nacylsarcosine derivative as a dispersing agent with the binders. The Examples show use of such a dispersing agent with a commercially available vinyl chloride-vinyl acetate-vinyl alcohol terpolymer.
U.S. Pat. No. 4,291,100 to Horigome et al. discloses a magnetic recording medium utilizing a polyoxyethylenesorbitane higher fatty acid ester surfactant. The Examples include use of such surfactants with vinyl chloride-vinyl acetate copolymers and vinyl chloride-vinyl acetate-vinyl alcohol terpolymers.
U S. Pat. No. 4,305,995 to Ota et al. shows a magnetic recording medium including a mixture of sorbitan mono-, di- and tri- higher fatty acid ester surfactants. The Examples show use of such surfactants with vinyl chloride-vinyl acetate copolymers.
U.S. Pat. No. 4,330,600 to Kawasumi et al. discloses a magnetic recording medium in which the dispersion characteristics of magnetizable particles are improved which results in improved saturation magnetic flux density and squareness ratio. These are achieved by treating the magnetizable particles with a titanium alcoholate compound having at least one group which is easily hydrolyzed and at least one oleophilic group which is hard to hydrolyze in an organic solvent. The examples show use of such titanium alcoholates with a commercially available vinyl chloride-vinyl acetate copolymer binder.
U.S. Pat. No. 4,400,435 to Yoda et al. notes that vinyl chloride-vinyl acetate copolymers have been used as binders, but that such copolymers do not have functional groups whereby it is easy to improve the dispersibility of magnetic powder in a magnetic layer. It is further said that it is not easy to carry out a thermosetting reaction. The use of vinyl chloride-vinyl acetate-vinyl alcohol copolymers instead had been proposed; and, because of the hydroxyl groups present, the dispersibility of magnetic powder is improved and the thermosettable reaction can be performed. However, because of the vinyl alcohol component, the glass transition temperature of the copolymer is disadvantageously high so as to provide difficulties in improving the surface properties by a calender process. A magnetic recording medium having improved orientation and maximum residual magnetic flux density is provided by using a vinyl chloride-vinyl acetate-maleic acid copolymer having a content of the maleic acid component of at least 1.5 percent.
In addition, there are available various vinyl chloride-vinyl acetate resins which are employed in magnetic recording media. For example, such resins include vinyl chloride-vinyl acetate-vinyl alcoholmaleic acid polymers in which the vinyl alcohol portion is obtained by hydrolysis.
J56077930-A(8133) discloses magnetic recording media in which a substrate is coated with a magnetic lacquer composed of ferromagnetic powder bonded with a binder containing a copolymer of 70-95 weight percent vinyl chloride and 1-25 weight percent hydroxyethylacrylate or hydroxyethylmethacrylate and 0-10 (meth)acrylic acid. It is stated that the squareness ratio and mass residual magnetic flux density are improved. When polyisocyanate (hardening agent) is added, the abrasion resistance is also improved.
U.S. Pat. No. 4,415,630 to Kubota et al. discloses a radiation-sensitive modified resin which is capable of being cross-linked by radiation. As one example, thermoplastic resins which can be effectively modified for radiation sensitivity include vinyl chloride-based copolymers such as vinyl chloride-vinyl acetate-vinyl alcohol copolymers, vinyl chloride-vinyl acetate-maleic acid copolymers, and vinyl chloride-vinyl acetate-end-OH-side-chain alkyl radical copolymers. Such thermoplastic resins which have one or more hydroxyl groups in the molecule can be reacted with a polyisocyanate compound, followed by reaction with a monomer having groups reactive to the isocyanate group and also having unsaturated double bonds that harden upon irradiation at a rate of one molecule of the former to one or more molecules of the latter. A further example involves a reaction product of the compound having at least one epoxy group in the molecule and a monomer having groups reactive with the epoxy group and also having radiation-setting, unsaturated double bonds. Specifically, an epoxy-containing thermoplastic resin, obtained by radical polymerization with glycidyl alcohol, is reacted with acrylic acid. By the ring-opening reaction of the carboxyl and epoxy groups, a resin is obtained which carries the acrylic double bonds as a pendant group.
Further, U.S. Pat. No. 3,755,271 to Montgomery discloses terpolymers of vinyl chloride, vinyl acetate and certain acrylic esters. These terpolymers can be employed alone or in admixture with other resins to make useful coatings. U.S. Pat. No. 3,884,887 to Montgomery provides a similar disclosure.
Also, East German DL140-463 discloses a crosslinkable binder for providing improved weather resistance, particularly to water, for various substrates. The composition comprises an acrylate ester, vinyl chloride, maleic acid or the half ester and one of the following: an epoxy containing monomer or (2) hydroxyethylacrylate or (3) a derivative of methylacrylamide.
Japanese Publication No. JA7203734-R discloses a composition which is said to provide good weathering properties and chemical resistance. The composition includes 2-6 weight percent of acrylic acid or maleic acid, 3-15 weight percent of glycidyl methacrylate and hydroxyethylacrylate, and 79-95 weight percent of an acrylate ester, alone or with other olefinically unsaturated monomers such as styrene or vinyl chloride.
Co-pending application Ser. No. 636,895 now abandoned, discloses novel phosphorylated compositions of a vinyl chloride copolymer or terpolymer having improved dispersion characteristics which are suitable for use in applications such as magnetic recording media. While such phosphorylated compositions provide improved dispersion characteristics, there still remains the need for improvement. More specifically, the phosphorylation reaction is not as facile as would be desired. Moreover, it is necessary to carefully carry out such reaction because of the inherent lack of thermal stability of such vinyl chloride compositions.
In addition, while vinyl chloride compositions are highly desirable for use as binder resins in magnetic recording media, it has long been recognized that compositions having improved thermal stability would be highly desirable. This is apparent because a wide variety of conditions must be met. Thus, binder resins must be storage stable under ambient conditions as well as being stable during a variety of processing operations in forming the various products, including grinding, drying and calendering operations. Likewise, the product must not unduly degrade in storage and must provide substantial durability in operation.
From the product performance standpoint, a high level of stability is desirable for such binder resins so that the magnetic recording medium performs adequately during its expected lifetime. The primary binder function of holding the particulate magnetic pigment on the base film through the rigors of production and use are critically dependent upon the molecular weight of such resins. Cumulative exposure to higher temperatures can reduce the molecular weight of vinyl resins, which results in premature mechanical failure. The obvious flaking of pigment and loss of information can be, of course, catastrophic. Degradation often manifests itself by low molecular weight products blooming to the surface with fouling of the heads or tape sticking to the guides.
Another failure mode results from the failure of the other components in the system due to the evolution of HCl from vinyl chloride resins. Thus, the polyester urethane elastomer binder ingredient often used in such media can itself degrade through acid-catalyzed hydrolysis. The loss of molecular weight and evolution of blooming species can cause the same problem as the primary degradation in the hardening polymer. In the extreme case, low levels of HCl could well be generated during long-term, still-frame playing which could corrode the head surfaces.
Accordingly, despite the considerable prior effort in this field, the need remains for binder systems utilizing vinyl chloride resins that can not only achieve the desired dispersion and magnetic characteristics over a widespread variety of usage requirements but can also provide relative freedom from thermal instability problems.